1. Field of the Invention
The present invention relates to a magnetic recording medium and, more particularly, to a magnetic recording medium having a coating layer composed mainly of an acicular magnetic powder and a binder having improved properties.
2. Description of the Prior Art
A magnetic recording medium has a magnetic coating layer which is formed on a non-magnetic substrate and which mainly consists of magnetic powder and a binder. When the magnetic characteristics of the magnetic recording medium are considered, they are considered to increase as the volume concentration of the magnetic powder in the magnetic coating layer increases. However, in practice, the magnetic coating layer is considered as a composite material of a binder, magnetic powder and so on and must have a certain strength. Therefore, a critical pigment volume concentration exists at which the strength of the magnetic coating layer becomes maximum. If the concentration of the pigment exceeds this critical pigment volume concentration, the binding force between the magnetic powder and the binder is generally weakened. If the magnetic recording medium in this state is used for recording/reproduction, the magnetic powder may peel off due to contact with a magnetic transducer head or tape guides, or the magnetic coating layer may be damaged or peeled off by a rotary head or a rotary drum. Then, the medium may not be put to use again.
A conventional binder is not sufficient in the binding force with the magnetic powder and provides the critical pigment volume concentration which is as low as 45 to 55%.
In order that a magnetic recording medium have excellent magnetic characteristics and electromagnetic transducing characteristics, the magnetic powder must be homogeneously dispersed within the magnetic coating layer. However, in the conventional magnetic recording media, dispersion of the magnetic powder is not necessarily satisfactory. For this reason, a dispersion agent is used to facilitate dispersion. When a dispersion agent is used, it is adsorbed in the magnetic powder to render its surface lipophilic. Then, the dispersibility of the mangetic powder in an organic solvent is improved and the orientation is facilitated, providing a magnetic recording medium with excellent magnetic characteristics or the like.
The lipophilic group of a surface active agent used as the dispersion agent, a typical example of which is an alkyl group having 7 to 21 carbon atoms, has a low surface energy and provides unsatisfactory binding force with the binder. Therefore, the critical pigment volume concentration at which the strength of the magnetic coating layer becomes maximum is 45 to 55% which is not satisfactory.